In recent years, from the viewpoint of protecting the global environment, vehicles are demanded to have low fuel consumption by reducing the weight thereof. In order to meet the demand, in the vehicles such as a two-wheeled vehicle and an all-terrain vehicle, as a frame member that constitutes a vehicle frame thereof, a frame member made of aluminum (Al) material has been employed. The frame member for use in a two-wheeled vehicle and an all-terrain vehicle requires a high tensile strength of 250 MPa or more. Thus, 7000 series Al alloy extruded materials (Al—Zn—Mg series) such as 7003 and 7N01, which are defined in the above-mentioned AA or JIS, have been conventionally used. Further, since the shape of the vehicle frame is complicated, the frame member that constitutes a final shape or at least one part of a structure of the vehicle frame for the two-wheeled vehicle and all-terrain vehicle is comprised of a plurality of Al members. The plurality of Al members is subjected to a fusion welding by welding processes such as TIG and MIG to be an integrated member. However, when a filler metal made of a 4000 series Al alloy (Al—Si series) such as 4043 or 4047 is used in the fusion welding, there is a problem in which the integrated Al members are fractured at a weld metal region or portion that has a low strength, because a weld metal has a low strength.
In order to improve the strength of the weld metal region formed in the fusion welding of the Al members made of 7000 series Al alloys, “KEIKINZOKU YOUSETSU” (Light metal welding), Vol. 45, No. 10, pp. 461-470 (2007) proposes a filler metal that is made of a 5000 series Al alloy (Al—Mg series) such as 5554, 5356, or 5183 for use in the fusion welding. However, for a further reduction in weight of the frame member for the two-wheeled vehicle and all-terrain vehicle, if the amount of Mg, Zn, or Cu in the 7000 series Al alloy, which provides the Al member constituting the frame member, is increased to reduce the thickness and improve the strength thereof, the conventional filler metal made of a 5000 series Al alloy cannot sufficiently lower the weld crack sensitivity. Thus, there is inherently problem in which the weld metal region has a lower strength than a heat-affected zone (HAZ) in a base material.
Further, as the 7000 series Al alloy has a high weld crack sensitivity, in order to reduce the weld crack sensitivity, JP-A-1-143791 proposes an Al alloy filler metal containing Mg: 6-10 wt % and Zr: 0.25-1.5 wt %. However, such a filler metal also inherently has various problems. Specifically, such a filler metal contains a large amount of Zr, so that a large crystallized product is formed in a metal structure. Thus, in the production of a filler metal having a diameter of 1.6 mm, 2.4 mm or the like, the large crystallized product adversely affects the wire drawing and causes a problem of cutting or the like in the middle of the wire drawing, which results in the reduction of productivity. In addition, the large crystallized product may be generated in the weld metal due to the presence of a large amount of Zr. Thus, characteristics of the weld joint may be unstable.